These three also happen to be the three fundamental principles expounded by Shaykh Muhammad ibn Abdul Wahhab in his book Thalaathatul-Usool, as they are the three questions one will be asked in the grave.
According to scholars, the three components which comprise the core of the Islamic message are Tawhid (Oneness of God), Risalah (Prophethood) and Ma'ad (Hereafter).